Method for fabricating negative electrode for secondary cell

ABSTRACT

The present invention relates to a method for readily producing an anode for rechargeable batteries having conflicting properties in good balance, including the corrosion resistance and the activities such as the initial activity and the high rate discharge performance, and having excellent recyclability. The method includes the steps of mixing and molding anode materials containing an electrically conductive material and at least two kinds of AB 5  type hydrogen storage alloys, wherein said alloys have substantially single phase structures and the same composition, wherein each of the alloys have an average crystal long axis diameter of 30 to 350 μm, and wherein the alloys have different ratios (D1/D2) of the average crystal long axis diameter (D1) to the average short axis diameter (D2).

FIELD OF ART

[0001] The present invention relates to methods for producing an anodefor rechargeable batteries, in particular, to methods for readilyproducing an anode for rechargeable batteries having corrosionresistance and activities, such as initial activity and high ratedischarge performance, in good balance.

BACKGROUND ART

[0002] AB₅ type hydrogen storage alloys have been the predominantmaterials for anodes of rechargeable batteries. For improved batteryperformance, the alloys are required to have various properties, such ashydrogen storage capacity, equilibrium pressure, corrosion resistance,and flatness of the plateau. Some of these properties conflict with eachother, so that studies have been made for improving one property withoutsacrificing the other. For example, studies have been made on additionalelements and compositions of the hydrogen storage alloys. However, theadditional elements increase the number of constitutional elements ofthe alloy, which adds to the difficulties in and the cost of the batteryrecycling, raising new problems.

[0003] For the purpose of improving the activities of hydrogen storagealloys, which is a factor in improved activities of the batteries, therehave been proposed to treat the alloy surface with acid or alkali, or toincrease the content of the A-site components. However, the activitiesof the alloy conflict with the corrosion resistance, and these methodsfor improving the activities thus simultaneously impair the corrosionresistance.

[0004] In the art of metal hydride-hydrogen batteries, electrode activematerials that satisfy both of these conflicting properties have beenunder development. To this end, there is proposed to mix an alloyexcellent in corrosion resistance and an alloy excellent in activities,for preparing starting materials for the active materials. However, suchalloys excellent in different properties used in this method are alsodifferent in their compositions or structures, or obtained by totallydifferent production methods. Thus, even though the activities and thecorrosion resistance are improved, the capacity and the internalpressure characteristics of the batteries are lowered, or the costs forrecycling the batteries are disadvantageously increased.

DISCLOSURE OF THE INVENTION

[0005] It is therefore an object of the present invention to providemethods for readily producing an anode for rechargeable batteries thatis excellent in recyclability and has both of the conflicting propertiesin good balance, in particular, the corrosion resistance and theactivities, such as the initial activity and the high rate dischargeperformance, simply by employing at least two kinds of particularhydrogen storage alloys having different crystal grain sizes.

[0006] According to the present invention, there is provided a methodfor producing an anode for rechargeable batteries comprising the stepsof mixing and molding anode materials comprising an electricallyconductive material and at least two kinds of AB₅ type hydrogen storagealloys,

[0007] wherein said at least two kinds of AB₅ type hydrogen storagealloys have substantially single phase structures and a samecomposition,

[0008] wherein each of said alloys have an average crystal long axisdiameter of 30 to 350 μm, and

[0009] wherein said alloys have different ratios (D1/D2) of an averagecrystal long axis diameter (D1) to an average crystal short axisdiameter (D2).

[0010] According to the present invention, there is also provided amethod for producing an anode for rechargeable batteries comprising thesteps of mixing and molding anode materials comprising an electricallyconductive material and anode active materials at least comprising:

[0011] AB₅ type hydrogen storage alloy (1) having substantially a singlephase structure and an average crystal long axis diameter of 30 to 350μm, wherein a ratio (D1/D2) of an average crystal long axis diameter(D1) to an average crystal short axis diameter (D2) is lower than 3, and

[0012] AB₅ type hydrogen storage alloy (2) having a same composition asthat of said alloy (1) and substantially a single phase structure,wherein a ratio (D1/D2) of an average crystal long axis diameter (D1) toan average crystal short axis diameter (D2) is not lower than 3.

PREFERRED EMBODIMENTS OF THE INVENTION

[0013] The present invention will now be explained in detail.

[0014] The methods of the present invention include the steps of mixingand molding anode materials containing an electrically conductivematerial and at least two kinds of AB₅ type hydrogen storage alloyshaving substantially single phase structures, the same composition, anddifferent crystal grain sizes, i.e. different D1/D2 ratios. As usedherein, the same composition means that the kinds of the compositionalelements are the same. (D1) represents the average long axis diameter ofthe crystals, and the long axis diameter is the maximum length of acrystal along its longitudinal axis. (D2) represents the average shortaxis diameter of the crystals, and the short axis diameter is theaverage length of four lines which intersect perpendicularly with theline segment representing the long axis diameter of a crystal at thepoints equally dividing the line segment into five, and are limited bythe grain boundary of the crystal grain.

[0015] The at least two kinds of hydrogen storage alloys used in thepresent invention are in the category of AB₅ type, all havingsubstantially single phase structures and the same composition. Whethera hydrogen storage alloy is of a single phase structure or not may beconfirmed by X-ray diffraction or under an electron microscope. Havingsubstantially a single phase structure herein means that the presence ofother phases cannot be observed clearly by these methods.

[0016] The AB₅ type hydrogen storage alloys used in the presentinvention each has the average crystal long axis diameter of 30 to 350μm, preferably 30 to 200 μm. Combinations of at least two hydrogenstorage alloys having different D1/D2 ratios may include a combinationof hydrogen storage alloy (1) having the D1/D2 ratio of lower than 3,preferably not higher than 2, and hydrogen storage alloy (2) of the samecomposition as that of alloy (1) having the D1/D2 ratio of not lowerthan 3, preferably not lower than 5. Such alloy combinations may becomposed of three or more kinds of alloys, and not limited to two kinds.The minimum D1/D2 ratio of hydrogen storage alloy (1) is notparticularly limited, and usually 1, whereas the maximum D1/D2 ratio ofhydrogen storage alloy (2) is not particularly limited, and usually 30,preferably 10.

[0017] When only hydrogen storage alloy (1) is used, or when the averagecrystal long axis diameter of hydrogen storage alloy (1) or (2) exceeds350 μm, the activities required for an anode active material ofrechargeable batteries may not be obtained. When only hydrogen storagealloy (2) is used, or when the average crystal long axis diameter ofhydrogen storage alloy (1) or (2) is less than 30 μm, the desiredcorrosion resistance is hard to be obtained.

[0018] The mixing ratio of the hydrogen storage alloys (1) and (2) asthe anode active materials may suitably be decided, and the ratio ofhydrogen storage alloy (1) to hydrogen storage alloy (2) by weight ispreferably 99:1 to 50:50, more preferably 95:5 to 80:20. When the mixingratio of hydrogen storage alloy (2) to hydrogen storage alloy (1) is toolow, sufficient improvement in initial activity may not be achieved, andwhen too high, sufficient improvement in corrosion resistance may not beachieved.

[0019] In the present invention, the compositions of the AB₅ typehydrogen storage alloys are not particularly limited, as long as thealloys fall under the AB₅ type and provide hydrogen storage capability.For example, the hydrogen storage alloys may have a compositionrepresented by the formula (1):

RNi_(x)M_(y)  (1)

[0020] wherein R stands for one or a mixture of rare earth elementsincluding yttrium, M stands for Co, Mg, Al, Mn, Fe, Cu, Zr, Ti, Mo, W,B, or a mixture thereof, x satisfies 3.3≦x≦5.3 and y satisfies0.1≦y≦1.5, with 4.7≦x+y≦5.5.

[0021] In the above formula, R may preferably be, for example, one ormore members selected from the group consisting of La, Ce, Pr and Nd. Inthe composition of R, the content of La may preferably be large so thatthe resulting active material exhibits high capacity. The La content maybe preferably not lower than 50%, more preferably not lower than 55%,most preferably not lower than 65%, by atomic percent. Thus thecomposition of R, when mainly composed of one or more members selectedfrom the group consisting of La, Ce, Pr, and Nd, is preferably selectedfrom 50 to 100 at % La, 0 to 50 at % Ce, 0 to 50 at % Pr, and 0 to 50 at% Nd.

[0022] In the above formula, M represents additional elements forcontrolling the hydrogen storage performance of the alloy. When thenumber of additional elements is too large, the inconveniences inrecycling the resulting alloy outstrip the contribution of theadditional elements to the alloy characteristics. Thus the number ofadditional elements is preferably 2 to 5, more preferably 2 to 3.

[0023] The hydrogen storage alloys used in the present invention may besubjected to surface coating by plating or with a high polymer, surfacetreatment with an acid or alkali solution, or any conventionaltreatment, for the purpose of further improving various propertiesbefore the alloys are processed into electrodes.

[0024] The hydrogen storage alloys used in the present invention may beprepared, for example, by melting alloy materials for preparing thealloy of the formula (1), cooling and solidifying the alloy melt intoflakes of a particular average thickness, and heat-treating the flakesunder particular conditions.

[0025] The average long axis diameter of the crystals of the alloy to beobtained may be controlled by regulating the cooling rate, the thicknessof the flakes, or the like factors in preparing the flakes. In general,the higher the cooling rate is, the smaller the crystal long axisdiameter is, and vice versa. Further, since the alloy in the form ofas-cast flakes does not have a single phase structure, the alloy flakesmay subsequently be heat-treated under the particular conditions forgiving a single phase structure thereto. If the cooling rate in theproduction of alloy flakes is too low, a secondary phase of crystalsappear, which grow so coarse that the alloy flakes cannot be made into asingle phase structure in the subsequent heat treatment, thus not beingpreferred. On the other hand, if the cooling rate is too high, thecrystals are made fine and readily made into a single phase structure,but the thickness of the alloy flakes is hard to be controlled withinthe particular range, and the productivity is lowered, thus not beingpreferred.

[0026] In view of the above, among the cooling conditions in producinghydrogen storage alloys (1) and (2), it is preferred to suitably selectthe cooling rate in preparing the alloy flakes from the range of 10 to3000° C. per second, taking the compositions into consideration.

[0027] In producing hydrogen storage alloy (1), the thickness of thealloy flakes is usually 0.1 to 0.5 mm, preferably 0.2 to 0.3 mm. If thealloy flakes are too thick, the temperature variation in the alloyflakes is great, which results in difficulty in generating crystals of auniform size. Too thick alloy flakes also provide enlarged reactionareas, which cause too much growth of the crystals in the subsequentlong-time heat treatment. Such alloy flakes may preferably be preparedby, for example, single- or twin-roll strip casing, centrifugal casting,or rotary disk casting.

[0028] On the other hand, in producing hydrogen storage alloy (2), whichhas a smaller average crystal long axis diameter than that of the alloy(1), the thickness of the alloy flakes is preferably adjusted to 0.05 to0.2 mm.

[0029] In producing hydrogen storage alloy (1), the heat treatment forgiving the alloy flakes a single phase structure is performed at 950 to1100° C. for 30 minutes to 10 hours. At lower than 950° C., it takes toomuch time for the crystals to grow to the predetermined crystal grainsize, resulting in dispersion in crystal grain size. At higher than1100° C., a secondary phase is reprecipitated, and the alloy of a singlephase structure may not be obtained.

[0030] In producing hydrogen storage alloy (2), the heat treatment forgiving the alloy flakes a single phase structure is performed at 900 to1000° C. for 1 to 10 hours. At lower than 900° C., the single phasestructure is hard to be given to the alloy flakes, and it takes time forthe crystals to grow to the predetermined crystal grain size. At higherthan 1000° C., the crystals may grow beyond the predetermined grainsize, or dispersion may occur in the crystal grain sizes.

[0031] In the methods of the present invention, at least two kinds ofAB₅ type hydrogen storage alloys are used as the anode active materials,which alloys have substantially single phase structures, differentcrystal grain sizes, and the same composition. As long as the desiredadvantages of the present invention are not substantially impaired, orin order to further improve such advantages, the anode active materialsmay optionally contain a hydrogen storage alloy of a slightly differentcomposition. The anode active materials may also contain an alloy suchas another hydrogen storage alloy that is inevitably contained.

[0032] In the methods of the present invention, the anode materialscontaining the above-mentioned anode active materials and anelectrically conductive material are mixed and molded.

[0033] The anode materials may optionally contain otherconventionally-used materials, as long as the desired advantages of thepresent invention are not impaired. Such other anode materials mayinclude, for example, known binders, electrical conductivity assistingagents, and the like. The mixing and molding may be performed inconventional manners, but the anode active materials are preferablymixed in advance.

[0034] According to the methods of the present invention, since theanode materials containing the electrically conductive material and theabove-mentioned at least two kinds of AB₅ type hydrogen storage alloysare mixed and molded, anodes for rechargeable batteries may be obtainedeasily, wherein the conflicting properties such as the activities andthe corrosion resistance are well balanced. In addition, since thecompositions of the active materials used in the methods may be madesubstantially the same, easily recyclable anodes for rechargeablebatteries may advantageously be obtained.

EXAMPLES

[0035] The present invention will now be explained in more detail withreference to Production Examples, Examples, and Comparative Examples,but the present invention is not limited to these.

Production Examples

[0036] <Production of Alloys>

[0037] Misch metal (abbreviated as Mm hereinbelow) manufactured bySantoku Corporation (rare earth composition: 70 at % La, 22 at % Ce, 2at % Pr, and 6 at % Nd), Ni, Co, Mn, and Al were mixed to have thecomposition shown in Table 1. The mixture was high-frequency melted inan alumina crucible in an argon gas atmosphere to obtain an alloy melt.The alloy melt was supplied via a tundish onto a single roll to berapidly cooled by strip casting, thereby obtaining flakes of a hydrogenstorage alloy. The alloy flakes were subjected to a heat treatment in aninert gas atmosphere. By suitably adjusting the cooling conditions ofthe alloy melt and the heat treatment conditions, alloys as shown inTable 2 were produced. The obtained hydrogen storage alloys weresubjected to observation of the alloy structures under a scanningelectron microscope, and X-ray diffraction to see whether the alloys hadsubstantially single phase structures. Further, from the alloystructures observed under the scanning electron microscope, (D1) and(D2) were determined. The results are shown in Table 2. TABLE 1 Name ofAlloy Mm Ni Al Co Mn AB_(x) A 1 3.50 0.25 0.60 0.35 4.70 B 1 3.55 0.300.80 0.35 5.00 C 1 4.30 0.25 0.60 0.35 5.50 D 1 3.30 0.25 0.60 0.35 4.50E 1 4.25 0.25 0.50 0.70 5.70

[0038] TABLE 2 Single Name of (D1) (D2) Phase or Alloy Alloy Composition(μm) (μm) (D1/D2) Not A-1 Alloy Composition 220 140 1.57 ◯ A-2 A inTable 1 151 33 4.58 ◯ A-3 60 25 2.40 X B-1 Alloy Composition 182 72 2.53◯ B-2 B in Table 1 83 16 5.19 ◯ C-1 Alloy Composition 310 260 1.19 ◯ C-2C in Table 1 264 86 3.07 ◯ D-1 Alloy Composition 322 288 1.12 X D-2 D inTable 1 43 11 3.91 X E-1 Alloy Composition 196 163 1.20 X E-2 E in Table1 68 9 7.56 X

Examples 1-4 and Comparative Examples 1-3

[0039] <Production of Electrodes>

[0040] Each of the hydrogen storage alloys prepared in ProductionExample were mechanically ground into hydrogen storage alloy powdershaving the average particle size of not larger than 60 μm. The alloypowders were mixed at the mixing ratio shown in Table 3 to obtain mixedpowders. 1.2 g of the mixed powders, 1 g of carbonyl nickel as anelectrically conductive material, and 0.2 g of fluororesin powders as abinder were mixed. The obtained mixture was wrapped with nickel mesh andpressure molded under the pressure of 2.8 ton/cm², to thereby obtain anelectrode of a hydrogen storage alloy.

[0041] <Measurement of Battery Performance>

[0042] Each of the electrodes thus produced was subjected to acharge-discharge test in 30% KOH in a pressure vessel under 5 atm forevaluating the initial activity, the high rate discharge performance,and the high corrosion resistance. The results are shown in Table 3.

[0043] The charge-discharge test was run for 10 cycles at the dischargecurrent of 0.2 C to evaluate the initial activity. Subsequently, to testthe high rate discharge performance, the capacity upon discharge at 1Con the 11th cycle was evaluated as the high rate discharge performance.The test was further run to test the corrosion resistance at thedischarge current of 0.2 C from the 12th cycle on, and the capacity onthe 500th cycle was evaluated as the high corrosion resistance. Thecapacities were evaluated as a percentage of the capacity upon chargeand discharge at 0.2 C on the 10th cycle, being 100. TABLE 3 High RateMixing Initial Discharge Corrosion Name of Ratio Activity PerformanceResistance Alloy (%) (%) (%) (%) Example 1 A-1 95 98.2 93.1 94.9 A-2 5Example 2 B-1 90 97.7 92.6 94.6 B-2 10 Example 3 C-1 85 96.8 94.3 95.4C-2 15 Example 4 B-1 60 93.6 91.5 92.5 B-2 40 Comp. Ex. 1 A-2 100 98.492.2 73.8 Comp. Ex. 2 D-1 90 88.3 82.6 53.6 D-2 10 Comp. Ex. 3 A-1 9090.1 89.5 68.2 E-2 10

1. A method for producing an anode for rechargeable batteries comprisingthe steps of mixing and molding anode materials comprising anelectrically conductive material and at least two kinds of AB₅ typehydrogen storage alloys, wherein said at least two kinds of AB₅ typehydrogen storage alloys have substantially single phase structures and asame composition, wherein each of said alloys have an average crystallong axis diameter of 30 to 350 μm, and wherein said alloys havedifferent ratios (D1/D2) of an average crystal long axis diameter (D1)to an average crystal short axis diameter (D2).
 2. A method forproducing an anode for rechargeable batteries comprising the steps ofmixing and molding anode materials comprising an electrically conductivematerial and anode active materials at least comprising: AB₅ typehydrogen storage alloy (1) having substantially a single phase structureand an average crystal long axis diameter of 30 to 350 μm, wherein aratio (D1/D2) of an average crystal long axis diameter (D1) to anaverage crystal short axis diameter (D2) is lower than 3, and AB₅ typehydrogen storage alloy (2) having a same composition as that of saidalloy (1), substantially a single phase structure, and an averagecrystal long axis diameter of 30 to 350 μm, wherein a ratio (D1/D2) ofan average crystal long axis diameter (D1) to an average crystal shortaxis diameter (D2) is not lower than
 3. 3. The method of claim 1,wherein said AB₅ type hydrogen storage alloys have a compositionrepresented by the formula (1): RNi_(x)M_(y)  (1) wherein R stands forone or a mixture of rare earth elements including yttrium, M stands forCo, Mg, Al, Mn, Fe, Cu, Zr, Ti, Mo, W, B, or a mixture thereof, xsatisfies 3.3≦x≦5.3 and y satisfies 0.1≦y≦1.5, with 4.7≦x+y≦5.5.
 4. Themethod of claim 2, wherein said (D1/D2) of the hydrogen storage alloy(1) is not higher than 2, and said (D1/D2) of the hydrogen storage alloy(2) is not lower than
 5. 5. The method of claim 2, wherein a ratio ofthe hydrogen storage alloy (1) to the hydrogen storage alloy (2) is 99:1to 50:50 by weight.
 6. The method of claim 2 wherein said AB₅ typehydrogen storage alloys have a composition represented by the formula(1): RNi_(x)M_(y)  (1) wherein R stands for one or a mixture of rareearth elements including yttrium, M stands for Co, Mg, Al, Mn, Fe, Cu,Zr, Ti, Mo, W, B, or a mixture thereof, x satisfies 3.3≦x≦5.3 and ysatisfies 0.1≦y≦1.5, with 4.7≦x+y≦5.5.